Tunnel Junction selector MRAM

ABSTRACT

A magnetoresistive random access memory (MRAM) cell includes a bottom electrode, a magnetic tunnel junction structure, a bipolar tunnel junction selector; and a top electrode. The tunnel junction selector includes a MgO tunnel barrier layer and provides a bipolar function for putting the MTJ structure in parallel or anti-parallel mode.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent is a continuation of U.S. application Ser. No. 16/589,255,filed on Oct. 1, 2019, which application is hereby incorporated byreference herein as if reproduced in its entirety.

BACKGROUND

In integrated circuit (IC) devices, magnetoresistive random accessmemory (MRAM) is an emerging technology for next generation non-volatilememory devices. MRAM is a memory structure including an array of MRAMcells. Each MRAM cell includes a magnetic tunnel junction (MTJ) element,and a resistance of the MTJ element is adjustable to represent logic “0”or logic “1”. The MTJ element includes one reference layer and oneferromagnetic free layer separated by a tunneling insulating layer. Theresistance of the MTJ element is adjusted by changing a direction of themagnetic moment of the ferromagnetic free layer with respect to that ofthe reference layer. The low and high resistances are utilized toindicate a digital signal “1” or “0”, thereby allowing for data storage.

From an application point of view, MRAM has many advantages. MRAM has asimple cell structure and CMOS logic comparable processes which resultin a reduction of the manufacturing complexity and cost in comparisonwith other non-volatile memory structures. Despite the attractiveproperties noted above, a number of challenges exist in connection withdeveloping MRAM. Various techniques directed at configurations andmaterials of these MRAMs have been implemented to try and furtherimprove device performance.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIGS. 1 through 14 illustrate intermediate steps in the formation of anMRAM cell, in accordance with some embodiments.

FIGS. 15 through 17 illustrate intermediate steps in the formation of anMRAM cell, in accordance with other embodiments.

FIGS. 18 through 23 illustrate intermediate steps in the formation ofembodiments of an MRAM cell with electrodes included in a pillar of theMRAM cell, in accordance with some embodiments.

FIGS. 24 through 34 illustrate intermediate steps in the formation ofembodiments of an MRAM cell using two pillar formation steps, inaccordance with some embodiments.

FIG. 35 illustrates an MRAM cell where the position of the tunneljunction selector and MTJ structure is changed, in accordance with someembodiments.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the invention. Specificexamples of components and arrangements are described below to simplifythe present disclosure. These are, of course, merely examples and arenot intended to be limiting. For example, the formation of a firstfeature over or on a second feature in the description that follows mayinclude embodiments in which the first and second features are formed indirect contact, and may also include embodiments in which additionalfeatures may be formed between the first and second features, such thatthe first and second features may not be in direct contact. In addition,the present disclosure may repeat reference numerals and/or letters inthe various examples. This repetition is for the purpose of simplicityand clarity and does not in itself dictate a relationship between thevarious embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly.

A cell of an MRAM device requires current to be able to flow in bothdirections. Read operations require passing a small current in a forwarddirection to measure resistance across the MRAM cell, while writeoperations require passing a larger current in both forward and reversedirections to control the spin direction of electrons in a free layer ina magnetic tunnel junction (MTJ) of the MRAM cell. For example, a MRAMbased memory device may use an access transistor to control the reversecurrent flow and ultimately the spin direction of the free layer of theMTJ of the MRAM cell. The access transistor can be switched using awrite word line. One end of the MRAM cell is connected to a bit line andthe other end of the MRAM cell is connected to a select line or readword line. This arrangement is known as a one transistor selector onemagnetic tunnel junction (1T-1MTJ) MRAM cell. Although, this arrangementprovides the ability to control the free layer spin, as cell sizesdecrease with improved fabrication technologies, the access transistorwould take a greater percentage of the foot print requirements for theMRAM cell.

Embodiments described herein eliminate the need for an accesstransistor. Instead a bipolar selector using a tunnel junction is used.Bipolar selectors generally, however, utilize unconventional materialsor noble metal contacts. Bipolar selectors also generally suffer fromlimited endurance (less than about 10⁶ cycles). Embodiment processes,however, utilize a tunnel junction using MgO in the bipolar selector.This advantageously results in a robust and economical bipolar selector.The resulting tunnel junction selector can endure a high number ofread/write accesses (greater than about 10¹⁶ cycles) and can sustainhigh current densities (greater than about 10 MA/cm²). Embodimentsdescribe an MRAM cell and device utilizing a one tunnel junction onemagnetic tunnel junction (1TJ-1MTJ) configuration, thereby eliminatingthe need for an access transistor. Embodiments use a bipolar selectormade from materials compatible with an MRAM cell and processescompatible with creating an MRAM devices and complementary metal oxidesemiconductor (CMOS) devices.

FIGS. 1 through 14 illustrate intermediate stages of the creation of anMRAM device 10 in accordance with some embodiments. FIGS. 15 through 17illustrate intermediate stages of the creation of an MRAM device 10 inaccordance with other embodiments. FIGS. 18 through 23 illustrateintermediate stages of the creation of an MRAM device 10 in accordancewith yet other embodiments. FIGS. 24 through 33 illustrate intermediatestages of the creation of an MRAM device 10 in accordance with stillother embodiments. FIG. 34 illustrates an MRAM device 10 followingformation processes in accordance with another embodiment. FIG. 35illustrates an MRAM device 10 where the formation order of features ischanged, in accordance with some embodiments.

In FIG. 1, in some embodiments, the substrate 100 may be a substrate andthe MRAM device 10 is formed on the substrate. MRAM device 10 mayinclude several MRAM cell areas, including MRAM cell 20 and MRAM cell25. After the layers of the MRAM cells of the MRAM device 10 are made,the cells are patterned into individual MRAM cells.

In some embodiments, the substrate 100 may be formed of a semiconductormaterial such as silicon, silicon germanium, or the like. In someembodiments, the substrate 100 is a crystalline semiconductor substratesuch as a crystalline silicon substrate, a crystalline silicon carbonsubstrate, a crystalline silicon germanium substrate, a III-V compoundsemiconductor substrate, or the like. In an embodiment the substrate 100may comprise bulk silicon, doped or undoped, or an active layer of asilicon-on-insulator (SOI) substrate. Generally, an SOI substratecomprises a layer of a semiconductor material such as silicon,germanium, silicon germanium, or combinations thereof, such as silicongermanium on insulator (SGOI). Other substrates that may be used includemulti-layered substrates, gradient substrates, or hybrid orientationsubstrates.

In some embodiments, the substrate 100 may be a carrier substratewithout any active devices formed therein, such as a glass carriersubstrate, a ceramic carrier substrate, or the like.

Redistribution structure 110 is formed over the substrate 100. In someembodiments, the redistribution structure 110 may be formed of aninsulating material 113, such as a dielectric material. In someembodiments, the redistribution structure 110 may include an Inter-MetalDielectric (IMD) layer or an Inter-Layer Dielectric (ILD) layer, whichmay include a dielectric material having a low dielectric constant (kvalue) lower than 3.8, lower than about 3.0, or lower than about 2.5,for example. The redistribution structure 110 may also includeconductive features, such as the conductive features 115. The insulatingmaterial 113 of the redistribution structure 110 may be formed of PSG,BSG, BPSG, FSG, TEOS, Black Diamond (a registered trademark of AppliedMaterials Inc.), a carbon-containing low-k dielectric material, HSQ,MSQ, or the like.

The conductive features 115 may be coupled to an active or passivedevice (e.g., a transistor or other electrical component) which may beembedded in the substrate 100 or the redistribution structure 110 orformed in another substrate. The conductive features 115 may include,for example, a source/drain region of a transistor, a gate electrode, acontact pad, a portion of a via, a portion of a metal line, and soforth. Active devices may comprise a wide variety of active devices suchas transistors and the like and passive devices may comprise devicessuch as capacitors, resistors, inductors and the like that together maybe used to generate the desired structural and functional parts of thedesign. The active devices and passive devices may be formed using anysuitable methods either within or else on the substrate 100 or theredistribution structure 110.

The conductive features 115 formed in the redistribution structure 110may include, for example, contacts or metal lines, which may be formedof copper or a copper alloy. In some embodiments the conductive features115 may be a part of an interconnect to provide addressing to the MRAMcells which will be formed in the MRAM device 10. In such embodiments,the conductive features 115 may be a control line, such as a bit line orword line. In some embodiments, conductive features 115 may includeother conductive materials such as tungsten, aluminum, or the like.Furthermore, conductive features 115 may be surrounded by a conductivediffusion barrier layer (not shown) formed underlying and encircling theconductive features 115. The conductive diffusion barrier layer may beformed of titanium, titanium nitride, tantalum, tantalum nitride, or thelike.

The conductive features 115 may be formed by any suitable process. Forexample, by a patterning and plating process where openingscorresponding to the conductive features 115 are made, the conductivediffusion barrier layer deposited in the openings (if used), followed bya seed layer. Next, the conductive features 115 are formed by anysuitable process, such as a plating process including electro-plating orelectroless-plating. Following the formation of the conductive features115, any excess material along with the excess seed layer and conductivediffusion barrier layer may be removed by suitable etching and/orpolishing process, such as by a chemical mechanical polishing (CMP)process. Other suitable processes may be used to form the conductivefeatures 115.

Redistribution structure 110 may include multiple layers of insulatingmaterial 113 and conductive features 115.

Also illustrated in FIG. 1, detail of a bottom via layer 110 b of theredistribution structure 110 is illustrated, in accordance with someembodiments. Bottom via layer 110 b may include an optional etch stoplayer 116, insulating layer 117, bottom electrode vias 119, and optionalnitrogen free anti reflective coating (NFARC) layer 118. Etch stop layer116 may be deposited over the redistribution structure 110, and may bemade of one or more layers. In some embodiments, etch stop layer 116 maycomprise a nitride, oxide, carbide, carbon-doped oxide, and/orcombinations thereof. In some embodiments, etch stop layer 116 may alsoinclude metal or semiconductor material, such as an oxide, nitride, orcarbide of a metal or semiconductor material. Such materials mayinclude, for example, aluminum nitride, aluminum oxide, silicon carbide,silicon nitride, silicon carbide, and the like. The etch stop layer 116may include multiple layers of the same or different material. Etch stoplayer 116 may be formed by any suitable method, such as by PlasmaEnhanced Chemical Vapor Deposition (PECVD) or other methods such asHigh-Density Plasma CVD (HDPCVD), Atomic Layer Deposition (ALD), lowpressure CVD (LPCVD), physical vapor deposition (PVD), and the like. Inaccordance with some embodiments, the etch stop layer 116 may also beutilized as a diffusion barrier layer for preventing undesirableelements, such as copper, from diffusing into a subsequently formedlayer. The etch stop layer 116 may be deposited to a total thickness ofbetween about 30 Å and about 100 Å, such as about 50 Å, though othervalues may be used and are contemplated.

Following depositing the etch stop layer 116, insulating layer 117 maybe formed using any suitable material by any suitable formation process.In one embodiment, the insulating layer 117 may include an insulatingmaterial, such as a silicon oxide network formed usingtetraethylorthosilicate (TEOS), tetramethylorthosilicate (TMOS), or thelike. The insulating layer 117 may be formed by any suitable process,such as by Plasma Enhance Chemical Vapor Deposition (PECVD),High-Density Plasma (HDP) deposition, or the like. In some embodiments,the insulating layer 117 may include silicon carbide, siliconoxynitride, or the like.

In some embodiments, the bottom via layer 110 b may include a NFARClayer 118 which may be formed to aide in a subsequent photo patterningprocess. The NFARC 118 may be formed using any acceptable process andmay include any suitable material. In some embodiments, a separate NFARC118 is not used and the insulating layer 117 may be used as a NFARC.

Next, the insulating layer 117 is patterned and openings are formed inthe insulating layer 117 and etch stop layer 116 to expose correspondingconductive features 115. The openings may be formed by any suitablemethod. For example, openings may be made in the NFARC layer, insulatinglayer 117, and the etch stop layer 116, and may be formed by aphoto-patterning process, using a patterned photo resist (not shown).The pattern of the patterned photo resist may be transferred to each ofthe layers by appropriate etching process using etchants selective tothe material of each layer. In some embodiments, the NFARC layer 118 (ifused) may act as a hard mask. In other embodiments, a separate hard mask(not shown) may be deposited over the NFARC layer 118 prior to etchingthe openings for the bottom electrode vias 119. After the conductivefeatures 115 are exposed by these openings, openings are then filledwith a conductive material to form bottom electrode vias 119.

In some embodiments, a conductive barrier layer (not shown) may beformed in the openings prior to filling the openings with the conductivematerial. The conductive barrier layer may be similar to that describedabove with respect to conductive features 115. In some embodiments, theconductive material of the bottom electrode vias 119 may overfill thevia openings and a subsequent planarization process, such as a chemicalmechanical polishing (CMP) process may be used to remove excessconductive material of the bottom electrode vias 119 and planarize thetop of the bottom electrodes vias 119 to the top of the NFARC layer 118.In embodiments which also use a conductive barrier layer to line the viaopenings, excess portions thereof which may be formed on the NFARC layer118 may also be removed by the planarization process. The bottomelectrode vias 119 may be deposited and planarized to a thickness ofabout 50 Å to about 500 Å, though other thicknesses are contemplated andmay be used.

The conductive material of the bottom electrode vias 119 may be formedby any suitable deposition process, such as by electro-plating,electroless plating, DC PVD, RFDC PVD, CVD, ALD, pulse DC, PVD, and thelike. Please note that the detail of the redistribution structure 110 inlayer 110 a and bottom electrode via layer 110 b are omitted in furtherdrawings.

In FIG. 2 a bottom electrode 125 of the MRAM device 10 is formed. Adifferent process for forming the bottom electrode 125 will be describedwith respect to FIG. 18, below. In some embodiments, the bottomelectrode 125 may be formed by first depositing an insulating layer 120,patterning the insulating layer 120 to form openings therein whichexpose the bottom electrode via 119, and then depositing the materialsof the bottom electrode 125 in the openings. The insulating layer 120may be made and patterned using materials and processes similar to thosediscussed above with respect to insulating material 113 which are notrepeated. In some embodiments, the bottom electrodes 125 may include asingle layer, while in other embodiments, the bottom electrodes 125 mayinclude multiple distinct layers of either the same material or ofdistinctive materials. In some embodiments, the bottom electrodes 125may include a single layer titanium nitride, tantalum nitride, nitrogen,titanium, tantalum, tungsten, cobalt, copper, or the like. In someembodiments, the bottom electrodes 125 may include a multi-layerstructure of titanium nitride, titanium, and titanium nitride; tantalumnitride, tantalum, and tantalum nitride; tantalum, tantalum nitride, andtantalum; titanium, titanium nitride, and titanium; tantalum andtitanium nitride; titanium and tantalum nitride; titanium nitride andtantalum nitride; titanium nitride and tungsten; tantalum nitride andtungsten; and so forth.

In FIG. 3, following the formation of the bottom electrodes 125 of theMRAM device 10, the magnetic tunnel junction (MTJ) structure 130 may beformed. The MTJ structure 130 may include any suitable configuration fora MTJ of an MRAM device, such as MRAM device 10. Various configurationsfor MTJ structure 130 are discussed with respect to FIGS. 4A and 4B.

Referring to FIGS. 4A and 4B, various example configurations MTJstructure are illustrated, in accordance with some embodiments. Itshould be understood that any suitable structure may be used for the MTJstructure 130.

In FIGS. 4A and 4B, the layers of the MTJ structure 130 may include anantiferromagnetic layer 132, a reference layer 134, and a free layer138. The antiferromagnetic layer 132 is sometimes referred to as asynthetic antiferromagnetic layer. The reference layer 134 is sometimesreferred to as a fixed layer. In FIGS. 4A and 4B, the MTJ structure 130also includes one or more tunnel barrier layers 136 disposed between thereference layer 134 and the free layer 138. In FIG. 4B, the MTJstructure 130 is inverted from that of FIG. 4A and is illustrated toinclude the tunnel barrier layer 136 interposed between the referencelayer 134 and the free layer 138. In addition, more or fewer layers ofthe MTJ structure 130 may be incorporated into the MRAM device 10.

In FIG. 4A, the antiferromagnetic layer 132 is formed on the bottomelectrode 125, the reference layer 134 is formed over theantiferromagnetic layer 132, and the free layer 138 is formed over thereference layer 134. However, other arrangements of the MTJ structure130 are contemplated, such as illustrated in FIG. 4B, where the freelayer 138 is formed on the bottom electrode 125, the reference layer 134is formed over the free layer 138, and the antiferromagnetic layer 132is formed over the reference layer 134. The antiferromagnetic layer 132,the reference layer 134, and the free layer 138 may be formedsequentially.

The antiferromagnetic layer 132 may be formed of a metal alloy includingmanganese (Mn) and another metal(s) such as platinum (Pt), iridium (Ir),ruthenium (Ru), rhodium (Rh), nickel (Ni), palladium (Pd), iron (Fe),osmium (Os), or the like. Accordingly, the antiferromagnetic layer 132may be formed of platinum manganese (PtMn), iridium manganese (IrMn),ruthenium manganese (RuMn), rhodium manganese (RhMn), nickel manganese(NiMn), palladium manganese (PdPtMn), iron manganese (FeMn), osmiummanganese (OsMn), alloys thereof, or the like. The reference layer 134and antiferromagnetic layer 132 may be formed of different materials orthe same materials. The reference layer 134 and free layer 138 may beformed of a ferromagnetic material alloy such as cobalt iron (CoFe),nickel iron (NiFe), cobalt iron boron (CoFeB), cobalt iron borontungsten (CoFeBW), or the like. The tunnel barrier layer 136 may beformed from magnesium oxide (MgO), aluminum oxide (Al₂O₃), aluminumnitride (AlN), aluminum oxynitride (AlON), hafnium oxide (HfO₂),zirconium oxide (ZrO₂), spinel alloys, such as spinel (MgAl₂O₄),chrysoberyl (BeAl₂O₄), gahnite (ZnAl₂O₄), galaxite (MnAl₂O₄),magnesiochromite (MgCr₂O₄), zincochromite (ZnCr₂O₄), combinationsthereof, or the like. It should be recognized that the various layers ofthe MTJ structure 130 may be formed of other materials. Theantiferromagnetic layer 132, reference layer 134, free layer 138, andtunnel barrier layer 136 may respectively be formed using any suitableprocess, for example, by PVD, DC PVD, RFDC PVD, CVD, ALD, pulse DC, andso forth and may be formed in single or multiple layers.

In FIG. 5 a hard mask 140 is formed over the MTJ structure 130. In asubsequent process, the hard mask 140 will be patterned and used as anetch mask in forming the pillars of the MRAM cells. The hard mask 140may be made of any suitable back-end-of-line material for metal hardmasks such as titanium nitride, tantalum nitride, or the like. In someembodiments, the hard masks 140 may be made of a composition whichincludes tantalum, tungsten, chromium, ruthenium, molybdenum, silicon,germanium, other MRAM compatible metals, or combinations thereof, suchas nitrides and/or oxides of these materials. The hard mask 140 may beformed using any suitable process, for example, by PVD, DC PVD, RFDCPVD, CVD, ALD, pulse DC, and so forth, to a thickness between about 10nm and 30 nm, though other thicknesses may be used.

In FIG. 6, a tunnel junction selector 150 is formed over the hard mask140. The tunnel junction selector 150 may serve as a bipolar selector toset electron spin of the free layer of the MRAM cell. The tunneljunction selector 150 may include multiple layers and have an overallthickness between about 10 nm to about 115 nm, depending on the tunneljunction selector 150, though other thicknesses may be used. The detailsfor the formation of the tunnel junction selector are described withrespect to FIGS. 7A and 7B.

The tunnel junction selector 150 acts as a bipolar selector by mimickingthe behavior of a Schottky diode. In other words, the tunnel junctionselector acts as a Schottky barrier. As such, forward bias voltageallows current to freely flow in the forward direction, while a reversebias voltage can temporarily overcome the barrier properties and allowcurrent to flow in the reverse direction. This property allows themagnetic spin of the free layer 138 of the MTJ structure 130 to be putinto parallel or anti-parallel mode with the reference layer 134, andthereby control the resistance associated with the MRAM cell.

In FIG. 7A, a tunnel junction selector 150 including three layers isillustrated. Bottom contact layer 154 of the tunnel junction selector150 may be made of any contact material which is suitable for use in anMRAM cell, including metals or semiconductors. For example, bottomcontact layer 154 may include tantalum, tungsten, chromium, ruthenium,molybdenum, silicon, germanium, other MRAM compatible metals, orcombinations thereof, such as nitrides and/or oxides of these materials.An MRAM compatible metal indicates a metal which is non-magnetic. Somemagnetic metals can be used as well, for example, an MRAM compatiblemetal may include, a cobalt iron boron alloy (CoFeB), where a largeboron content (greater than about 50% by weight) would render the alloynon-magnetic.

Bottom contact layer 154 may be deposited using any suitable method, forexample, by PVD, DC PVD, RFDC PVD, CVD, ALD, pulse DC, and so forth, toa thickness between about 5 nm and 20 nm, though other thicknesses maybe used.

Following the deposition of the bottom contact layer 154, the tunnellayer 155 may be formed over the bottom contact layer 154. The tunnellayer 155 may comprise magnesium oxide (MgO) and may be between about0.5 nm and about 5 nm thick or between about 0.5 nm and about 3.5 nmthick, such as about 1.5 nm thick. With thicker MgO (greater than about2.5 nm) and bias, partly due to the inclusion of a greater series withthe MTJ structure, the transport through direct tunneling through MgOand through interface state tunneling becomes more significant,indicating a greater contribution of tunneling current. The tunnel layer155 is thin enough that electrons are able to tunnel through the tunnellayer 155 when a biasing voltage is applied on the MRAM cell, such asMRAM cell 25. Thicknesses beyond 5 nm are impractical due to a greatercontribution of current needed to achieve tunneling as the tunnel layer155 thickness increases. Thicknesses less than 0.5 nm may be ineffectiveto achieve the Schottky barrier properties needed in a bipolar selector.The tunnel layer 155 may be formed using any suitable method, forexample, by PVD, DC PVD, RFDC PVD, CVD, ALD, pulse DC, and so forth.

As noted above, using MgO as the material of the tunnel layer 155 hasthe advantage of providing a robust material which can endure a highnumber of read/write cycles (e.g., forward bias and reverse biasalternating states). Moreover, the MgO material can sustain highercurrent densities than other materials so that a thicker MgO materiallayer may be used as necessary to achieve suitable Schottky behavior.

In one embodiment, a PVD process is used to deposit the tunnel layer 155on the bottom contact layer 154. The tunnel layer 155 may be grown at ornear room temperature, for example, between about 15° C. to about 40°C., though other temperatures may be used and are contemplated. Theresulting tunnel layer 155 includes an MgO layer exhibiting strongcrystallinity, i.e. having a dominant crystalline orientation andstructure.

Following the deposition of the tunnel layer 155, a top contact layer156 is formed. The top contact layer 156 may be formed using processesand materials similar to those discussed above with respect to thebottom contact layer 154.

Following the deposition of the top contact layer 156 the tunneljunction selector 150 may be annealed by a low temperature anneal, forexample, between about 200° C. to about 400° C., for about 1 second toabout 180 minutes, though other temperatures and anneal times arecontemplated. During this anneal, the crystallinity of the MgO of thetunnel layer 155 is transferred to the contacting metal or semiconductormaterial electrode layers—the bottom contact layer 154 and top contactlayer 156.

In FIG. 7B, a tunnel junction selector 150 including five layers isillustrated, in accordance with some embodiments. In these embodiments,the contact layers (the top contact layer 156 and bottom contact layer154 of FIG. 7A) are replaced with a two layer structure, including acontact layer and an interlayer between the contact layer and the tunnellayer 155. When an n-type or p-type semiconductor, such as silicon orgermanium, respectively, is used as the top contact layer 156 or bottomcontact layer 154, an implicit Schottky barrier is formed because thereis a depletion region in the semiconductor material. Especially wherecertain metal materials are used in the top contact layer 156 or bottomcontact layer 154, however, an interlayer between the contact layer andthe tunnel layer 155 promotes non-linearity of the IV curve. Withoutsuch non-linearity, there is a risk that the tunnel junction selector150 would simply add in series with the MTJ structure 130, and thereforedegrade the magneto-resistive properties of the MRAM device 10.

Bottom contact layer 152 of FIG. 7B may be deposited using materials andprocesses similar to the bottom contact layer 154 of FIG. 7A. Bottomcontact layer 152 may be deposited to a thickness between about 5 nm toabout 50 nm, though other thicknesses are contemplated and may be used.

Interlayer 153 may be formed over the bottom contact layer 152.Interlayer 153 may be an ultrathin oxide or nitride layer, between about1 nm and about 5 nm. In some embodiments, interlayer 153 may includetitanium oxide (TiO₂), hafnium oxide (HfO₂), silicon oxide (SiO₂), andso forth. In some embodiments, interlayer 153 may include titaniumnitride (TiN), hafnium nitride (HfN), silicon nitride (Si_(x)N_(y)), andso forth. Other suitable materials may be used. The interlayer 153 maybe formed using any suitable techniques. In some embodiments, forexample, the interlayer 153 may be formed in situ by deposition of ametal or semiconductor material with an oxygen treatment or a nitrogentreatment, such as deposition in an oxygen ambient environment,deposition in an oxygen enriched environment, deposition followed by anoxygen plasma treatment or exposure to natural oxygen for spontaneousoxidation, or deposition with a nitridization process. The interlayer153 may be formed using any suitable method, for example, by PVD, DCPVD, RFDC PVD, CVD, ALD, pulse DC, and so forth.

In other embodiments, the interlayer 153 may be formed by scavengingoxygen atoms from a nearby source. For example, during or afterformation of the tunnel layer 155 in FIG. 7 b, discussed below, atomsfrom the MgO material may be incorporated into a bottom contact layer,such as bottom contact layer 152, to convert some of the bottom contactlayer into the interlayer 153. In some embodiments, an interlayer metalor interlayer semiconductor material may be deposited separate from thebottom contact layer 152, which is then converted to an oxide byscavenging oxygen from the tunnel layer 155.

The tunnel layer 155 may be formed over the bottom contact layer 152using materials and processes similar to those discussed above withrespect to the tunnel layer 155 of FIG. 7A. As noted above, in someembodiments the tunnel layer 155 may be formed directly on the bottomcontact layer 152 and the interlayer 153 subsequently formed fromscavenged oxygen atoms from the MgO of the tunnel layer 155. In otherembodiments, the tunnel layer 155 may be formed directly on theinterlayer 153.

Interlayer 157 may be formed over the tunnel layer 155. Interlayer 157may be formed using processes and materials similar to those discussedabove with respect to interlayer 153. In particular, interlayer 157 maybe a deposited metal or semiconductor material which is in situ oxidizedthrough an oxygen treatment. Interlayer 157 may be a silicon oxidenetwork formed using a suitable deposition technique for silicon oxide.Interlayer 157 may also be formed using scavenged oxygen atoms from thetunnel layer 155 to oxidize a metal or a semiconductor material, such asa portion of the top contact layer 158 after it is formed or aninterlayer of a metal or semiconductor material. Interlayer 157 may alsobe a deposited metal or semiconductor material which is combined withnitrogen using a nitridization process.

In some embodiments, either one or both of interlayer 153 and interlayer157 may be utilized. Where both interlayer 153 and interlayer 157 areutilized, they may be the same material or different materials. Also,where both interlayer 153 and interlayer 157 are utilized they may beformed by the same or different techniques.

The top contact layer 158 may be formed using processes and materialssimilar to those discussed above with respect to top contact layer 156(or bottom contact layer 154) of FIG. 7A. The top contact layer 158 maybe deposited to a thickness between about 5 nm to about 50 nm, thoughother thicknesses are contemplated and may be used.

Following formation of the top contact layer 158, the tunnel junctionselector 150 may be annealed using processes and conditions similar tothose discussed above with respect to FIG. 7A. During this anneal, thecrystallinity of the MgO layer may be transferred to the interlayers,interlayer 153 and interlayer 157, and may be transferred to the topcontact layer 158 and bottom contact layer 152. In the same anneal or ina separate anneal process, in some embodiments, oxygen from the tunnellayer 155 may diffuse into the interlayer 153 or bottom contact layer152 to form an oxide of the interlayer 153 or of the bottom contactlayer 152 (which then results in the interlayer 153). Similarly, oxygenfrom the tunnel layer 155 may diffuse into the interlayer 157 or topcontact layer 158 to form an oxide of the interlayer 157 or of the topcontact layer 158 (which then results in the interlayer 157).

In FIG. 8, a hard mask 160 is formed over the tunnel junction selector150. Hard mask 160 may be formed using materials and processes similarto those discussed above with respect to hard mask 140. In a subsequentprocess, the hard mask 160 will be patterned and used as an etch mask informing the pillars of the MRAM cells. In some embodiments, the hardmask 160 is made of different materials than the hard mask 140. In otherembodiments, such as discussed below with respect to FIGS. 25 through34, the same material may be used, since the respective underlyinglayers are etched in different steps.

In FIG. 9, the hard mask 160 is patterned to protect the MRAM cells,such as MRAM cell 20 and MRAM cell 25 during patterning the tunneljunction selector 150 and hard mask 140. Following patterning the tunneljunction selector 150 and hard mask 140, the hard mask 140 is also usedin patterning the MTJ structure 130. The patterning of the hard mask 160may be done by any suitable photo patterning technique.

In FIG. 10, each of the underlying layers is etched in successive etchsteps to form the 1TJ-1MTJ pillars 170 corresponding to the MRAM cell 20and MRAM cell 25 of MRAM device 10. The etching of each of the layers ofthe tunnel junction selector 150, hard mask 140, and MTJ structure 130may be performed using a suitable etchant selective to the particularlayer being etched. The etching technique may include reactive ionetching (RIE), ion beam etching (IBE), or the like. Etching may beperformed using process gases selected from Cl₂, N₂, CH₄, He,CH_(x)F_(y), SF₆, NF₃, BCl₃, O₂, Ar, C_(x)F_(y), HBr, or thecombinations thereof, depending on the particular material being etched.N₂, Ar and/or He may be used as carrier gases. For example, for etchingtitanium, titanium nitride, tantalum, tantalum nitride, or the like inhard mask layer 38, Cl₂ may be used, along with other gases such as thecarrier gas.

FIG. 11 illustrates the formation of dielectric capping layer 220 inaccordance with some embodiments. In accordance with some embodiments,dielectric capping layer 220 is formed of silicon nitride, siliconoxynitride, or the like. The formation process may be a CVD process, anALD process, a Plasma Enhance CVD (PECVD) process, or the like.Dielectric capping layer 220 may be formed as a conformal layer.

FIG. 12 illustrates a gap-filling process, in which dielectric material230 is filled into the gaps between the 1TJ-1MTJ pillars 170. Dielectricmaterial 230 may be a TEOS formed oxide, PSG, BSG, BPSG, USG, FSG,SiOCH, flowable oxide, a porous oxide, or the like, or combinationsthereof. Dielectric material 230 may also be formed of a low-kdielectric material. The formation method may include CVD, PECVD, ALD,FCVD, spin-on coating, or the like.

In FIG. 13, after the gap-filling process, a planarization process suchas a CMP process or a mechanical grinding process may be performed. Theplanarization process may be performed using hard mask layer 160 as aCMP stop layer. Accordingly, the top surface of dielectric material 230may be level with the top surface of hard mask 160. In otherembodiments, the dielectric capping layer 220 or a top electrode layer255 may be used as CMP stop layer. Embodiments where the dielectriccapping layer 220 is used as the CMP stop layer are illustrated withrespect to FIGS. 15-17, 22, and 35. Embodiments where the top electrodelayer 255 is used as the CMP stop layer are illustrated with respect toFIG. 23.

In FIG. 14, top electrodes 255 are formed and top electrode vias 265 areformed. The top electrodes 255 may be formed using processes andmaterials similar to the bottom electrodes 125 as discussed above withrespect to FIG. 2 or bottom electrodes 125 discussed below with respectto FIG. 18. In particular, top electrodes 255 may be laterallysurrounded by an insulating layer 250. Following the formation of thetop electrodes 255, an insulating layer 260 may be deposited, openingsformed therein to expose the top electrodes 255, and top electrode vias265 deposited therein. The materials and processes used to forminsulating layer 260 may include those discussed above with respect toinsulating layer 117 of FIG. 1. Similarly, the materials and processesused to form the top electrode vias 265 may include those discussedabove with respect to bottom electrode vias 119 of FIG. 1.

Following the formation of the top electrode vias 265, anotherredistribution structure may be formed over the top electrode vias 265to couple MRAM cells together into arrays and provide inputs to the MRAMcells to bias the MRAM cells. The redistribution structure may be formedusing processes and materials similar to those discussed above withrespect to redistribution structure 110.

The resulting MRAM device 10 may include multiple MRAM cells, such asMRAM cell 20 and MRAM cell 25. The MRAM cell 20 and MRAM cell 25 may beconnected in an array, so that their top or bottom electrodes areelectrically coupled to each other. The MRAM cell 20 may also beconnected to another MRAM cell (not shown) so that the other of the topor bottom electrode is coupled to the other MRAM cell.

Because the tunnel junction selector 150 is placed in series with theMTJ structure 130 in a single pillar, cell size of the MRAM cells may bedecreased. As a result, the spacing between MRAM cells may also bedecreased and the density of the MRAM device may increase. In someembodiments, the cell size may be less than 8F2, and may be between 6F2and 4F2, inclusive.

FIGS. 15 through 17 include an illustration where the planarizationprocess following FIG. 12 uses the dielectric capping layer 220 as CMPstop layer. FIG. 15 illustrates the flow following FIG. 12, where aplanarization process removes a portion of the top surface of dielectricmaterial 230 and levels the top surface of the dielectric material 230with the top surface of the dielectric capping layer 220.

In FIG. 16, a patterning process may be used to form a mask 240 over thedielectric material 230 and create openings 241, removing by a suitableetching process a portion of the dielectric capping layer 220 over thehard mask 160 to expose the hard mask 160. The hard mask 160 mayoptionally also be removed. A portion of the dielectric capping layer220 extends above the hard mask 160, which will partially surround thesubsequently formed top electrodes 255.

In FIG. 17, top electrodes 255 may be formed using any suitable process.In some embodiments, one or more metal layers using materials such asthose discussed above with respect to bottom electrodes 125 may bedeposited. These materials may then be removed as needed to keep theportion of the top electrodes 255 aligned with each of the 1TJ-1MTJpillars 17o. Then, an insulating layer 250 may be formed around the topelectrodes 255 to laterally encapsulate the top electrodes 255. Theinsulating layer 250 may be formed using processes and materials such asdiscussed above with respect to the insulating layer 120. In anotherembodiment, the insulating layer 250 may be formed as a layer, thenopenings formed therein corresponding to the top electrodes 255. In suchembodiments, the mask 240 of FIG. 16, may be formed over the insulatinglayer 250 with openings 241 extending through the insulating layer 250and dielectric capping layer 220. Then, the top electrodes 255 maysubsequently be formed in the openings 241. After the top electrodes 255and insulating layer 250 are formed, they may be planarized by agrinding or CMP process to level the top surface of the top electrodes255 with the top surface of the insulating layer 250.

Top electrode vias 265 may be formed using processes and materialssimilar to those discussed above with respect to FIG. 14. Following theformation of the top electrode vias 265, another redistributionstructure may be formed over the top electrode vias 265 to couple MRAMcells together into arrays and provide inputs to the MRAM cells to biasthe MRAM cells. The redistribution structure may be formed usingprocesses and materials similar to those discussed above with respect toredistribution structure 110.

FIGS. 18 through 23 illustrate certain intermediate stages in theformation of a MRAM device 10, in accordance with some embodiments. Inthese described embodiments, one or both of the bottom electrode 125 ortop electrode 255 may be formed as a layer extending across the lateralextents of the work area, such as the lateral extents of the substrate100 or the lateral extents of the layer immediately underneath thebottom electrode 125 or top electrode 255.

The intermediate process illustrated of FIG. 18 follows that of FIG. 8as discussed above. As illustrated in FIG. 18, however, the bottomelectrode 125 is formed as one or more layers which are patterned in alater patterning process. Each of the one or more layers of the bottomelectrodes 125 may be formed using any suitable materials and processes,including materials and processes similar to those discussed above withrespect to the bottom electrodes 125 of FIG. 2. The one or more layersof the bottom electrodes 125 may extend to the lateral extents of theredistribution structure 110 and lateral extents of the substrate 100.

In FIG. 18, one or more layers for the top electrodes 255 are formedover the hard mask 160. The one or more layers for the top electrodes255 may be formed using any suitable processes and materials, includingthose used for the formation of the one or more layers of the bottomelectrodes 125.

In FIG. 19, a mask 210 is formed over the one or more layers for the topelectrodes 255 and patterned to protect the areas of the underlyinglayers which will form the 1TJ-MTJ pillars of MRAM device 10.

In FIG. 20, the 1TJ-MTJ pillars 171 are formed by an etching processusing mask 210 as an etch mask. Each of the layers is etched in turnusing a suitable etching process and materials, such as those discussedabove with respect to FIG. 10.

In FIG. 21, the dielectric capping layer 220 is formed over the 1TJ-MTJpillars 171 using processes and materials similar to those discussedabove with respect to FIG. 11. Here, however, the dielectric cappinglayer 220 extends along sides of the bottom electrodes 125 and topelectrodes 255.

In FIG. 22, a gap-filling process is used to deposit dielectric material230 over and in the gaps between the 1TJ-MTJ pillars 171. Dielectricmaterial 230 may be formed using any suitable processes and materials,including those discussed above with respect to the dielectric material230 described above with respect to FIG. 12. Following the deposition ofthe dielectric material 230, a planarization process, such as a CMPprocess or grinding process may level the top surface of the dielectricmaterial 230 with the top surface of the dielectric capping layer 220(or top surface of the top electrode 255 as shown in FIG. 23).

Top electrode vias 265 are then formed. An insulating layer 260 may bedeposited over the dielectric material 230 and patterned to formopenings over the top electrodes 255. Top electrode vias 265 may beformed in the openings. The insulating layer 260 and top electrode vias265 may be formed using any suitable processes and materials, includingthose discussed above with respect to insulating layer 117 and bottomelectrode vias 119 of FIG. 1. As illustrated in FIG. 22, the topelectrode vias 265 are partially surrounded by the dielectric cappinglayer 220, as they extend through a neck of the dielectric capping layer220.

FIG. 23 illustrates that the planarization process after the gap-fillprocess depositing the dielectric material 230 uses the top electrode255 as CMP stop, so that the top surface of the dielectric material 230is level with the top surface of the top electrodes 255. Next, theinsulating layer 260 and top electrode vias 265 may be formed using anysuitable processes and materials, including those discussed above withrespect to insulating layer 117 and bottom electrode vias 119 of FIG. 1.As illustrated in FIG. 23, the bottoms of the top electrode vias 265 arealso level with the top of the dielectric capping layer 220.

Following the formation of the top electrode vias 265, anotherredistribution structure may be formed over the top electrode vias 265to couple MRAM cells together into arrays and provide inputs to the MRAMcells to bias the MRAM cells. The redistribution structure may be formedusing processes and materials similar to those discussed above withrespect to redistribution structure 110.

FIGS. 24 through 34 illustrate embodiments where the MTJ structure 130is patterned to form MTJ pillars 172 (see FIG. 25) prior to forming thetunnel junction selector 150 (see FIG. 28).

FIG. 24 picks up in the process described above after FIG. 3 or afterFIG. 5, in other words, after the formation of the MTJ structure 130.Optionally, the hard mask 140 may also be formed, and although it is notillustrated in FIG. 24, the formation and patterning of hard mask 140 isillustrated and described above.

In FIG. 24 a mask layer 310 is formed over the MTJ structure 130 toprotect areas of the MTJ structure 130 which are formed into MTJ pillars172 (see FIG. 25). The mask layer 310 may be formed and patterned usingany acceptable process, such as a photo patterning process. In someembodiments, the mask layer 310 may be a photo patternable material,while in other embodiments, the mask layer 310 may be an oxide ornitride which is patterned using a photo-patternable material, which issubsequently removed.

In FIG. 25, the mask layer 310 is used to pattern the MTJ structure 130to form MTJ pillars 172. The MTJ pillars 172 can be patterned using anysuitable processes and materials, including processes and materialsdescribed above with respect to the patterning of the 1TJ-1MTJ pillars170 of FIG. 10.

In FIG. 26, a dielectric capping layer 320 is formed over the MTJpillars 172. The dielectric capping layer 320 may be formed using anysuitable materials and processes, including the materials and processesdescribed above with respect to the formation of the dielectric cappinglayer 220 of FIG. 11.

In FIG. 27, a gap-fill process is used to deposit the dielectricmaterial 330 in the gaps between MTJ pillars 172. The gap-fill processmay use any suitable processes and materials, including the processesand materials described above with respect to the formation of thedielectric material 230 of FIG. 12. The gap-fill process may cause thedielectric capping layer 320 to extend over the MTJ pillars 172. Aplanarization process, such as a CMP process or grinding, may be used tolevel the top surface of the dielectric material 330 with the tops ofthe MTJ pillars 172. In embodiments which include the hard mask 140, thetop surface of the hard mask 140 may be used as a CMP stop for theplanarization process, so that the top surface of the dielectricmaterial 330 is level with the top surface of the hard mask 140.

In FIG. 28, if not already formed, the hard mask 140 may be formed overthe dielectric material 330 and the MTJ pillars 172. In someembodiments, using the hard mask 140 in the patterning of the MTJpillars 172, the hard mask 140 may have suffered damage. In suchembodiments, the hard mask 140 may be removed and reformed, or anadditional hard mask 140 may be over the damaged hard mask layer torestore it.

After the formation of the hard mask 140, the tunnel junction selector150 may be formed over the hard mask 140. Following the formation of thetunnel junction selector 150, the hard mask 160 may be formed over thetunnel junction selector 150.

The hard mask 140, tunnel junction selector 150, and hard mask 160 maybe formed using processes and materials similar to those discussed abovewith respect to their respective counterparts in FIGS. 5 through 8.

In FIG. 29, in another patterning process, the tunnel junction selector150 may be patterned to form TJ pillars 173 for each of the MRAM cells,such as MRAM cells 20 and 25. The TJ pillars 173 can be patterned usingany suitable processes and materials, including processes and materialsdescribed above with respect to the patterning of the 1TJ-1MTJ pillars170 of FIG. 10. In particular, a mask layer may be formed over thepillars to be patterned to protect them from the patterning process.

In FIG. 30, a capping dielectric layer 340 is deposited over the TJpillars 173. The capping dielectric layer 340 may be formed using anysuitable materials and processes, including the materials and processesdescribed above with respect to the formation of the dielectric cappinglayer 220 of FIG. 11.

In FIG. 31, a gap-fill process is used to deposit a dielectric material350 in the gaps between TJ pillars 173. The gap-fill process may use anysuitable processes and materials, including the processes and materialsdescribed above with respect to the formation of the dielectric material230 of FIG. 12. The gap-fill process may cause the dielectric material350 to extend over the TJ pillars 173.

In FIG. 32, a planarization process, such as a CMP process or grinding,may be used to level the top surface of the dielectric material 350 withthe tops of the TJ pillars 173. In some embodiments, the hard mask 160may be used as a CMP stop so that the top surface of the hard mask 160is level with the top surface of the dielectric material 350. In otherembodiments, the capping dielectric layer 340 may be used as the CMPstop, while in yet other embodiments, a top electrode may be used as aCMP stop, so that each of these respective CMP stops has a top surfacelevel with the top surface of the dielectric material 350.

In FIG. 33, top electrodes 255 may be formed followed by top electrodevias 265. The top electrodes 255 may be laterally encapsulated ininsulating layer 250 and the top electrode vias 265 may be laterallyencapsulated in insulating layer 260. The top electrodes 255, topelectrode vias 265, insulating layer 250, and insulating layer 260 maybe formed using materials and processes such as those described abovewith respect to their respective counterparts in FIG. 14.

Following the formation of the top electrode vias 265, anotherredistribution structure may be formed over the top electrode vias 265to couple MRAM cells together into arrays and provide inputs to the MRAMcells to bias the MRAM cells. The redistribution structure may be formedusing processes and materials similar to those discussed above withrespect to redistribution structure 110.

In FIG. 34, embodiments are illustrated which include patterning thebottom electrode 125 along with the MTJ pillar 172′. Similarly, FIG. 34also illustrates embodiments which include patterning the top electrode255 along with the MTJ pillar 173′, so that the 1TJ-1MTJ pillar 174′includes both the MTJ pillar 172′ and TJ pillar 173′. The formation ofthe bottom electrode 125 and/or top electrode 255 in this manner may beperformed using materials and processes discussed above with respect totheir respective counterparts in FIG. 22.

One should understand that the described embodiments may be mixed sothat the formation of the bottom electrode 125 is, for example,consistent with embodiments illustrated in FIG. 34, while the topelectrode 255 is, for example, consistent with embodiments illustratedin FIG. 23 or 33, and vice versa.

In FIG. 35, the structure of FIG. 14 is illustrated, except that theorder of forming the tunnel junction selector 150 and the MTJ structure130 is reversed so that the MTJ structure 130 is over the tunneljunction selector 150. One will understand that the MTJ structure 130and the tunnel junction selector 150 may be formed in either order forany of the embodiments discussed above.

Embodiments provide a bipolar tunnel junction selector in line with aMTJ structure to provide the ability to set the MTJ structure intoparallel or anti-parallel mode. The tunnel junction selector uses MgO asthe material of the tunnel barrier, thereby providing robust operationand ease of manufacture with materials and processes which arecompatible with MRAM and CMOS devices. In the tunnel junction selector,an insulating interlayer can be disposed between the tunnel barrier andthe top contact and/or between the tunnel barrier and the bottomcontact, to promote non-linearity of the IV curve. Advantages alsoinclude reducing the complexity of the MRAM device by eliminating anaccess transistor and other circuit logic to support switching theaccess transistor, thereby reducing MRAM cell size.

One embodiment is a device including a first magnetoresistive randomaccess memory (MRAM) cell including a bottom electrode, a magnetictunnel junction structure, a bipolar selector including a tunneljunction, and a top electrode. In an embodiment, the bipolar selectorincludes a bottom contact layer, a tunnel junction layer including MgO,and a top contact layer. In an embodiment, the bottom contact layer ortop contact layer includes: tantalum, tungsten, chromium, ruthenium,molybdenum, silicon, germanium, or a non-magnetic alloy of CoFeB. In anembodiment, the bottom contact layer and the top contact layer share asame crystallinity as the tunnel junction layer. In an embodiment, thetunnel junction layer has a thickness between 0.5 nm and 5 nm. In anembodiment, the bipolar selector includes a bottom contact layer, atunnel junction layer, a first interlayer interposed between the bottomcontact layer and the tunnel junction layer, and a top contact layer. Inan embodiment, the first interlayer includes an oxide. In an embodiment,the bottom contact layer includes a first material, and the firstinterlayer is an oxide of the first material. In an embodiment, bipolarselector of the device further includes a second interlayer interposedbetween the top contact layer and the tunnel junction layer.

Another embodiment is a device including a first memory cell, the firstmemory cell including a magnetic tunnel junction disposed in a firstpillar, a bipolar selector disposed in the first pillar, where thebipolar tunnel junction selector includes a tunnel junction Schottkybarrier, a top electrode over the first pillar, and a bottom electrodeunder the first pillar. The device includes a second memory cellconnected with the first memory cell by the top electrode. The devicealso includes a third memory cell connected with the first memory cellby the bottom electrode. A dielectric material fill laterally surroundsthe first pillar. In an embodiment, the bipolar selector includes abottom contact, a first interlayer on the bottom contact, a tunnel layerincluding MgO with a thickness between 0.5 nm and 5 nm on the firstinterlayer, a second interlayer on the tunnel layer; and a top contacton the second interlayer. In an embodiment, the top contact and bottomcontact have the same crystalline structure as the tunnel layer. In anembodiment, the first interlayer and the second interlayer includedifferent material compositions. In an embodiment, the first interlayerand second interlayer are each between 1 nm and 5 nm thick.

Another embodiment is a method including depositing a bottom electrodelayer over a substrate. A magnetic tunnel junction (MTJ) structure isformed over the bottom electrode layer. A bipolar selector is formedover the MTJ structure, the bipolar selector including a tunneljunction. A top electrode layer is deposited over the bipolar selector.A masking layer is patterned over the top electrode layer. Portions ofthe MTJ structure and portions of the bipolar selector are etched toform one or more pillars. A gap-fill material is deposited laterallysurrounding a first pillar of the one or more pillars. In an embodiment,forming the bipolar selector includes depositing a bottom contact layer,depositing a tunnel junction layer, and depositing a top contact layer.In an embodiment, forming the bipolar selector further includes forminga first interlayer interposed between the bottom contact layer and thetunnel junction layer, and forming a second interlayer interposedbetween the top contact layer and the tunnel junction layer, the firstinterlayer including an oxide. In an embodiment, forming the firstinterlayer includes depositing a first material and oxidizing the firstmaterial by introducing oxygen into a chamber containing the firstinterlayer, depositing the first material and oxidizing the firstmaterial by scavenging oxygen atoms from the tunnel junction layer, orscavenging oxygen atoms from the tunnel junction layer to oxidize aportion of the bottom contact layer. In an embodiment, etching to formone or more pillars includes, prior to forming the bipolar selector,etching the MTJ structure to form a MTJ portion of the one or morepillars, the MTJ structure including the MTJ portion; and etching theportions of the bipolar selector to form a tunnel junction selectorportion of the pillars, further including: forming a first dielectriccapping layer over the MTJ portion of the one or more pillars, andforming a second dielectric capping layer over the tunnel junctionselector portion of the pillars. In an embodiment, the method furtherincludes etching the bottom electrode layer to form a bottom electrodeafter forming the top electrode layer.

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

What is claimed is:
 1. A device comprising: a magnetoresistive randomaccess memory (MRAM) cell comprising: a bottom electrode; a magnetictunnel junction structure electrically coupled to the bottom electrode;a bipolar selector comprising, in order: a first contact layer, a firstinterlayer, a tunnel junction layer, a second interlayer, and a secondcontact layer, the bipolar selector being aligned with the magnetictunnel junction structure in a single pillar, the bipolar selectorelectrically coupled to the magnetic tunnel junction structure; and atop electrode electrically coupled to the bipolar selector.
 2. Thedevice of claim 1, wherein the tunnel junction layer of the bipolarselector comprises MgO.
 3. The device of claim 1, wherein the firstinterlayer or second interlayer comprises an oxide or nitride ofhafnium, silicon, or titanium.
 4. The device of claim 1, wherein thefirst contact layer and the second contact layer each comprise anon-magnetic conductive material or alloy.
 5. The device of claim 4,wherein the non-magnetic conductive material or alloy comprises iron. 6.The device of claim 1, wherein the first contact layer, firstinterlayer, tunnel junction layer, second interlayer, and second contactlayer each share the same crystallinity.
 7. The device of claim 1,wherein the bipolar selector is configured to mimic a behavior of aSchottkey diode.
 8. A device comprising: a magnetoresistive randomaccess memory (MRAM) cell comprising: a bottom electrode; a magnetictunnel junction structure electrically coupled to the bottom electrode;a bipolar selector comprising a tunnel junction, the bipolar selectorbeing aligned with the magnetic tunnel junction structure in a singlepillar, the bipolar selector laterally surrounded by an insulatingmaterial, the bipolar selector electrically coupled to the magnetictunnel junction structure; and a top electrode electrically coupled tothe bipolar selector.
 9. The device of claim 8, wherein the bipolarselector has electrical properties of a Schottky diode.
 10. The deviceof claim 8, wherein the bipolar selector comprises: a bottom contact;the tunnel junction over the bottom contact; and a top contact over thetunnel junction.
 11. The device of claim 10, wherein the bipolarselector further comprises: a first interlayer between the bottomcontact and the tunnel junction; and a second interlayer between the topcontact and the tunnel junction.
 12. The device of claim ii, wherein thefirst interlayer and the second interlayer each comprise an oxide ornitride of silicon, hafnium, or titanium.
 13. The device of claim 10,wherein the bottom contact and the top contact each comprise anon-magnetic conductive material.
 14. The device of claim 8, wherein thetunnel junction of the bipolar selector comprises MgO.
 15. A methodcomprising: forming a bottom electrode over a conductive feature in asubstrate; forming a magnetic tunnel junction (MTJ) structure over thebottom electrode; forming a bipolar selector over the MTJ structure, thebipolar selector comprising a tunnel junction; and depositing a firstinsulating layer laterally surrounding the bipolar selector.
 16. Themethod of claim 15, wherein forming the MTJ structure comprises:depositing functional layers of the MTJ structure; and patterning thefunctional layers of the MTJ structure to form the MTJ structure in apillar.
 17. The method of claim 15, wherein forming the bipolar selectorcomprises: depositing a bottom contact layer; depositing a tunneljunction layer; depositing a top contact layer; and patterning thebottom contact layer, tunnel junction layer, and top contact layer in apillar vertically aligned with the MTJ structure.
 18. The method ofclaim 17, further comprising: annealing the bottom contact layer, tunneljunction layer, and top contact layer, the annealing causing acrystallinity of the bottom contact layer and a crystallinity of the topcontact layer to align to a crystallinity of the tunnel junction layer.19. The method of claim 17, wherein forming the bipolar selector furthercomprises: forming a first interlayer interposed between the bottomcontact layer and the tunnel junction layer, wherein forming the firstinterlayer comprises: depositing a first material and oxidizing thefirst material by introducing oxygen to the first interlayer, depositingthe first material and oxidizing the first material by scavenging oxygenatoms from the tunnel junction layer, or scavenging oxygen from thetunnel junction layer to oxidize a portion of the bottom contact layer;and forming a second interlayer interposed between the top contact layerand the tunnel junction layer.
 20. The method of claim 15, furthercomprising: after forming the MTJ structure and prior to forming thebipolar selector, etching the MTJ structure to form a MTJ pillar; anddepositing a second insulating layer laterally surrounding the MTJpillar.